1. Field
This disclosure relates generally to semiconductor devices, and more specifically, to semiconductor devices that have a stressor.
2. Related Art
Semiconductor devices have been found to have improved performance by including one or more stressors. The additional stress improves performance by increasing carrier mobility. For example, adding compressive stress to the channel of a P channel transistor has been shown to improve its performance. Similarly, adding tensile stress to the channel of an N channel transistor has been found to improve its performance. The stressor types can be categorized as those that are formed over the transistor features such as over the source/drains and/or gates and those that are embedded. The embedded types are typically at least part of the source/drains where stress can be applied to the channel. Although the desirability of embedded stressors is known, the methods of achieving them can result in problems or less stress than should be possible. For example, stress can be achieved using silicon/germanium and silicon/carbon lattice structures in conjunction with lattice structures that are just silicon. The amount of stress can generally be increased by increasing the concentration of the germanium or the carbon. This, however, can be difficult to achieve and the problems with germanium and carbon are not necessarily the same.
Accordingly it is desirable to provide a method for achieving embedded stressors that overcome or improve upon one or more of these issues.